Strategic information systems planning (SISP) is a critical challenge for organizations. Some researchers have sug- gested that more incremental SISP in an uncertain environment produces greater planning success, while others have suggested that more comprehensive SISP does so in that environment. The purpose of this study was to test the effect of incremental versus comprehensive SISP on SISP success in environments of varying uncertainty. A questionnaire defined SISP in terms of character- istics of incremental and comprehensive planning. It measured environmental uncertainty in terms of 1) the changeability and unpredictability components of dynamism, 2) heterogeneity, and 3) the scarcity and competition components of hostility. It assessed planning success as a second-order construct composed of alignment, analysis, cooperation, and capabilities. A postal survey collected data from 161 IS executives. The constructs were extensively validated. In general, greater SISP comprehensive- ness predicted greater SISP success. Greater changeability and unpredictability, however, weakened the impact of such SISP on success. On the other hand, as the environment became more competitive, more comprehensive SISP led to greater SISP suc- cess. These findings contribute by suggesting that planners should expect comprehensive SISP to be less effective as changeability and unpredictability increase, but more effective as competition increases.